My intentions? To define a \pm command that takes two arguments.
The first argument needs to be used as a reference itself in order to obtain another variable: \currentValue.
Then, this variable (\currentValue) is appended to some other text ('test') in order to create a new label.

Things look good, but for some reason, when the final label is created, an \hbox {} is appended to the end of the test\currentValue label.

Any idea what the problem is? Or how can I achieve my goal, if not in this manner?

\ref{key} is a command to print something. If can contain boxes or font commands. It is normally not suitable as part of the key of a label. You could try the zref package. With it you can define reference commands which gives back "pure" values.
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Ulrike FischerJun 19 '12 at 13:23